In this Timeline Prince Edward, fourth son of King George III, is infertile and can't impregnate his wife Princess Victoria of Saxe-Coburg-Saalfeld. After Prince Edward dies in 1820 he had left no issue to become the next in line to gain the throne (in OTL he had a daughter, Princess Alexandrina Victoria, the later Queen Victoria I.) This meant that the heir to the throne at that point became Prince Ernest Augustus, who was also to become the next King of Hanover.
Then in 1837 King William IV of the United Kingdom and Hanover passed away, and Prince Ernest Augustus ascended both the throne of Hanover and the United Kingdom of Great Britain and Ireland. He became King Ernest Augustus I of both nations, though he preferred Hanover to Britain and lived in Hanover for much of his reign, leaving a council of high officials to look after the islands in his absence. This led to the 'Augustian Era' in the United Kingdom.